1. Field of the Invention
The present invention relates to a channel estimator, demodulator, speed estimator and method thereof, and more particularly to a channel estimator, demodulator, speed estimator and method thereof for determining a speed of a mobile device.
2. Description of the Related Art
A mobile communication system may employ a multiple access technology, such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and/or Code Division Multiple Access (CDMA).
A CDMA communication system may transfer data signals of each of its mobile stations within a single frequency bandwidth range using a Pseudo Noise (PN) code. The PN code may be used to distinguish between mobile stations. A receiver (e.g., a mobile station, a base station, etc.) may use a PN code matching a PN code of a corresponding transmitter in order to decode data sent from the corresponding transmitter. However, while the receiver and the transmitter may communicate if the same PN code is used by the receiver and the transmitter, the data sent from the transmitter to the receiver may not be decoded properly if the respective PN codes are not synchronized.
At the transmitter side, a data signal to be transmitted to the receiver may be spread. In an example, the spreading may include a data signal with n bits and may be multiplied by a spreading code with n bits to generate a spread signal. The spread signal may be transmitted to the receiver.
At the receiver side, the spread signal received from the transmitter may be de-spread. In an example, the spread signal may include n bits and may be multiplied by a de-spreading code with n bits to recapture the original data signal.
Conventional CDMA communication systems may employ a frequency bandwidth spreading technique. CDMA technology may typically be used in digital cellular system, a Personal Communication System (PCS), and/or an International Mobile Telecommunication-2000 (IMT-2000). Analog cellular systems may typically be referred to as first generation systems and digital systems (e.g., Global System for Mobile Communication (GSM), a Plasma Driven Catalyst (PDC), a IS-95, a IS-136, etc.) may typically be referred to as second generation systems.
The first and second generation systems may spread a wireless voice communication and may include services (e.g., a Short Message Service (SMS), access to data networks, etc.). Third generation systems may be designed for multi-media communication, may provide a higher quality picture and/or a higher quality video and may provide a higher data rate of communication. Accordingly, third generation systems may be widely used in public/private wireless communication networks.
An example of a third generation system may be a Wideband-CDMA (W-CDMA) system. The W-CDMA system may adopt a 32 kbps Adaptive Differential Pulse Code Modulation (ADPCM) and may be capable of maintaining a call connection for mobile stations moving at higher speeds. The W-CDMA system may employ a Direct Spread (DS) method that may be efficient at higher frequencies and may have lower interference/fading characteristics.
The term ‘fading’ may indicate that a strength of a received electric wave or signal may be changing (e.g., getting weaker) at a higher rate. The term ‘fast fading’ may indicate that a strength of an arriving signal may be changing at a higher rate based on an individual delay location. For example, fading may occur when the receiver moves at higher speeds. Fast fading may reduce an ability of the receiver to restore or decode data received from the transmitter without errors.
W-CDMA systems may include a rake receiver which may perform a synchronized sampling on a bandwidth spreading signal, correlation detection for a multiple path delay and weighting diversity for a channel reflection. The rake receiver may include a plurality of fingers and may demodulate higher energy path signals (e.g., the 3 or 4 path signals having the highest energy) among a plurality of received multi-path signals. Each of the fingers of the rake receiver may determine channel characteristics based on the higher energy path signals in order to perform channel compensation. Data in transmissions received at the receiver from the transmitter may be recovered by combining the higher energy path signals.
A channel estimator may determine receiving performance (e.g., of a Direct Spread/Code Division Multiple Access (DS/CDMA) system). Receiving performance may be affected by phase distortion, for example due to Rayleigh Fading or fast fading of a mobile wireless channel.
In DS/CDMA systems, a pilot channel with a plurality of pilot symbols may be transferred after being code-divided. The pilot channel may be used for a downward link so as to perform synchronization detection. A condition of a communication channel may be estimated by observing the pilot channel. Knowledge of the condition of the communication channel may reduce receiving errors due to fading (e.g., fast fading).
The channel estimator may be adapted for use in conventional CDMA modems and may be implemented using an Finite Impulse Response (FIR) filter, an Infinite Impulse Response (IIR) filter or a hybrid of the FIR filter and the IIR filter.
However, because of implementation complexity, the hybrid of the FIR filter and the IIR filter may be more difficult to use in the implementation of the channel estimator. As a result, the single FIR filter and the single IIR filter may typically be used for implementing the channel estimator.
The channel estimator employing the single FIR filter may have a processing delay occupying half of a channel observation duration required for channel estimation. A buffering process may thereby be required on a data channel performing channel compensation. Further, as the spreading factor increases, the problem of processing delay may likewise increase. Higher processing delays due to channel estimation may also delay speed power control in CDMA systems, thereby reducing performance of a feedback-loop power control.
Adjusting channel observation duration with regard to the speed of a mobile object may be difficult with conventional methodologies. As such, a fixed channel observation duration may be used, and a real system environment may operate at lower speeds. Thus, a fixed coefficient of the FIR filter may be based on the lower speeds.
In a system environment operating at higher speeds, estimating performance and receiving performance may be degraded because the filter coefficient may be configured for system environments operating at lower speeds.
CDMA systems may implement the channel estimator using the IIR filter. If the channel estimator is implemented using the IIR filter, an order of the IIR filter may be increased to obtain a sharp frequency response. There may be a trade-off between performance of the IIR filter and an ease of its implementation since implementation complexity may increase as the order of the IIR filter increases.
The channel estimator may be implemented using the IIR filter configured for operation with lower speed system environments. Therefore, in system environments operating at higher speeds and employing the IIR filter, an ability to estimate performance and receiving efficiency of a communication channel may be degraded. As mobilities of mobile stations continue to increase (e.g., due to faster forms of transportation), the problems associated with higher speed mobile stations in communication systems may likewise increase.